During operation of an internal combustion engine, hydrocarbon fuel and oxygen burn in the presence of nitrogen. The fuel is converted principally into carbon dioxide and water, creating extremely high gas pressures that displace pistons to produce engine power. This combustion also results in the formation of contaminants. These contaminants include soot, which is formed from incomplete combustion, as well as organic, sulfur-based and nitrogen-based acids. Each contaminant causes engine wear, increased oil viscosity and unwanted deposits when introduced into lubricating oil through contact with the lubricant in the cylinder bore or in blow-by gases.
One method for controlling combustion by-products has been to include additives, such as detergents and dispersants, in the lubricating oils to interact with the contaminants. For example, additives can be employed to inhibit agglomeration of sludge and soot, and thereby minimize the formation of viscosity-increasing materials. Additives may also be employed to neutralize combustion acids to minimize corrosive wear.
There are, however, limitations to the use of additives for combustion by-product control. During normal operation of an engine, combustion acids deplete additives through the formation of salts that render their protective properties ineffective. Before additive exhaustion, it is necessary to drain and replace the lubricant.
Further, additives have upper concentration limits in commercial lubricant formulations. Beyond a certain concentration, detergents themselves can add to piston deposits. At high concentrations, dispersants can increase viscosity especially at low temperatures because they have a higher molecular weight than oil. The additive concentration upper limit in commercial lubricants thus determines the intervals between oil drains.
Frequent oil drains have both direct and indirect consumer costs, as well as environmental impact. For each oil drain, consumers bear the direct costs of a new filter and lubricant, mechanic labor, and in the case of commercial trucks, lost delivery time. Consumers bear the indirect costs of filter and lubricant recycle or disposal. They also endure the negative environmental impact associated with the inappropriate disposal of engine oil. Extended oil drain intervals accordingly conserve valuable resources.
In order to reduce emissions, engine manufacturers have begun employing a technology known as Exhaust Gas Recirculation (“EGR”). This technology recycles exhaust back into the combustion chamber. Acids and soot particles that would otherwise be emitted to the atmosphere instead enter the lubrication system through the boundary layer of lubricant in the piston chamber and via blow-by gases. Thus, while EGR may improve emissions, it produces an increased load of soot and acid in the oil, and eventually may lead to a decrease in oil drain intervals due to the limitations on additive concentrations that may be employed in lubricating oils.
Another method for controlling combustion by-products has been to include a chemical filtration medium in oil filters that is capable of capturing the by-products and/or replenishing lubricating oil additives as oil cycles through the filters. For example, Brownawell, et al. in U.S. Pat. No. 4,906,389, U.S. Pat. No. 5,068,044, U.S. Pat. No. 5,069,799, U.S. Pat. No. 5,164,101 and U.S. Pat. No. 5,478,463, teach disposing strong base materials in an oil filter to immobilize combustion acids transported to the oil filter in the form of a combustion acid-weak base complex. Soluble weak bases, commonly referred to as dispersants, are typically employed in commercial lubricants to help neutralize combustion acids and control viscosity increase. The weak bases and combustion acids interact to form soluble neutral salts that travel within the lubricating oil from the piston ring zone of an internal combustion engine to the oil filter. A strong base material immobilized in the oil filter displaces the weak base from the complex, thereby immobilizing the combustion acids in the oil filter and recycling the weak base to neutralize subsequently produced combustion acids. In effect, there is an ion exchange whereby the strong base disposed in the oil filter exchanges with the weak base in the combustion acid-weak base complex. As a result, the weak base is regenerated and recycled with the lubricant to neutralize additional acid. The immobilization of the combustion acids and the reuse of detergent and dispersant allows an increase in the time between oil drains.
The Brownawell, et al. examples teach the use of strong bases such as calcium carbonate, magnesium carbonate, magnesium oxide and zinc oxide, among others. While the teachings of Brownawell, et al. provided a positive contribution to the arts, the disclosures fail to indicate any understanding of the strong base's morphology and its impact upon exchange kinetics and capacity. Applicants of the present invention, including common inventor Darrell W. Brownawell, have since discovered that not all strong base materials are created equal with respect to their ability to immobilize combustion acids and control viscosity increase.
For example, it has been discovered that the exchange between the weak base-combustion acid complex and the strong base is to a large degree an irreversible surface phenomenon under engine operating conditions. Thus, the more surface area available for this exchange, the higher the capacity of the strong base to immobilize combustion acids. A non-porous material comprising a strong base accordingly will have only its external surface area available for acid immobilization. In comparison, a highly porous material may have an increased amount of surface area, since it has internal as well as external surface area. Additionally, applicants of the present invention have determined that a portion of the surface area may not be available for the exchange due to the physical dimension of the weak base.
If the combustion acid-weak base complex is too large to enter a pore, then a strong base associated with that pore effectively is unavailable to displace the weak base and to capture the combustion acid. Pores must be large enough to accept the complex. Pores may also be too large, whereby the particle structural integrity is compromised. For example, the pores may collapse during the manufacturing and/or handling of the material, or when exposed to fluid pressure as oil is circulated through a filter containing the material.
The inventors of the above-listed patents identify only one specific strong base material—Catalyst 75-1 from ICI/Katalco. As discussed below, this material provides a limited amount of usable surface area for accepting combustion acid-weak base complexes.
The zinc oxide adsorbent Catalyst 75-1 scavenges hydrogen sulfide (H2S) from sour gas production and its high capacity derives from a high surface area engineered to capture this small molecule. While it does function in the lubrication application described in the patents above, its suitability is far from ideal. Hydrogen sulfide has a small cross-sectional diameter (<5 Å) and pores that allow its free diffusion may be much too small to adsorb the combustion acid-weak base complexes (believed to have a mean cross-sectional diameter of approximately 60 Å) occurring in a lubrication system.
Although Catalyst 75-1 is no longer manufactured, its usable surface area may be calculated from information occurring in the open literature. Using published values for pore volume (see, e.g., U.S. Pat. No. 4,717,552) and pore diameter measured using mercury intrusion porosimetry (“Application of Three-Dimensional Stochastic Pore Network to Zinc Oxide Particle” S. Javad-Mirrezaei Roudaki, Dissertation for the degree of Master of Science, Dept. of Chemical Engineering, University of Manchester Institute of Science and Technology, February 1989), the total usable surface area of Catalyst 75-1 for this application may be initially calculated to be approximately 40 m 2/gm. However, catalyst 75-1 is a spherical formed particle and due to well-documented shielding, ink bottle, and skin effects (see, e.g., “Analytical Methods in Fine Particle Technology” Webb, P. A., Orr, C;. Micromeritics Instrument Corp.; Norcross, Ga.; 1997, pp 172-173; Catalysis Today, 18 (1993) 509-528; and The Canadian Journal of Chemical Engineering, 83 (2005) 1-5), mercury porosimetry overestimates its surface area. Electron micrographs of samples with low melting point alloy intrusion (see “Application of Three-Dimensional Stochastic Pore Network to Zinc Oxide Particle” S. Javad-Mirrezaei Roudaki, Dissertation for the degree of Master of Science, Dept. of Chemical Engineering, University of Manchester Institute of Science and Technology, February 1989; “Applications of Visualized Porosimetry for Pore Structure Characterization of Adsorbents and Catalysts” The 1994 ICHEME Research Event, J. Mirrezaei-Roudaki, A. AlLamy, R. Mann, A. Holt, 1994) clearly show the presence of voids in this material that range from one to seven microns. These voids are not present in the mercury intrusion data, but may account for a minimum of 50% of the total intrusion volume. In addition, macroscopic cracks and voids account for up to another 15% of the total intrusion volume. These large voids contribute less than one m2/gm of usable surface area to the total surface area. A summary of the Applicant's calculations, based on the above discussion, is shown in Table 1 below.
TABLE 1Usable Surface Area of Catalyst 75-1 determined by Mercury IntrusionPorosimetry and Low Melting Point Alloy IntrusionVtotal,Dpore,Atotal,Comment(cm3/gm)(Angstroms)Constant(m2/gm)aIncorrectly ignoring “shielding” and0.30c 300d440“ink bottle” effectsbSubtracting volume due to 1-7 micron0.15300420voids (50% of pore volume compriseslarge voids)eSubtracting volume due to 1-7 micron0.105300414voids and cracks (65% of pore volumecomprises large voids)eRemaining pores with diameters greater0.15-0.19510,000  4 0.6-0.78than ca. 1 micron contribute negligibleusable surface areaSurface area accessible to weak base-15-21combustion acid complex withincatalyst 75-1Table Notes:aCalculations of total surface area using Washburn's Equation model, A = 4V/Db“Analytical Methods in Fine Particle Technology,” Webb, P.A., Orr, C., Micromeritics Instrument Corp., Norcross, GA, 1997, pp 172-73cPore Volume = 0.30 cm3/gm, typical of Catalyst 75-1 (see U.S. Pat. No. 4,717,552)dAverage Pore Diameter = 300 Å, typical of catalyst 75-1 (see “Application of Three-Dimensional Stochastic Pore Network to Zinc Oxide Particle” S. Javad - Mirrezaei Roudaki, Dissertation for the degree of Master of Science, Dept. of Chemical Engineering, University of Manchester Institute of Science and Technology, February 1989)eVolume of micron sized pores, see electron micrographs of Low Melting Point Alloy Intrusion in Catalyst 75-1 (see “Application of Three-Dimensional Stochastic Pore Network to Zinc Oxide Particle” S. Javad - Mirrezaei Roudaki, Dissertation for the degree of Master of Science, Dept. of Chemical Engineering, University of Manchester Institute of Science and Technology, February 1989; “Applications of Visualized Porosimetry for Pore Structure Characterization of Adsorbents and Catalysts” The 1994 ICHEME Research Event, J. Mirrezaei-Roudaki, A. AlLamy, R. Mann, A. Holt, 1994
Thus, the usable surface area of Catalyst 75-1 for this application conservatively falls within the range of 15-21 m2/gm, when macroscopic void volume is properly taken into account. A surface area larger than 21 m2/gm derived from pores sufficiently sized to accept combustion acid-weak base complexes would enable the exchange capacity to be maximized and oil drain intervals to be lengthened.
In light of the foregoing, what is still needed is a chemical filter comprising a strong base material having increased usable surface area that is capable of efficiently immobilizing combustion acids and controlling viscosity increase.